Fly Ash and GGBS in Concrete
Supplementary cementitious materials (SCMs) are used across Indian concrete construction to partly replace cement. This page explains what fly ash and GGBS are, typical replacement levels, and what to check before using them.
What SCMs are
Supplementary cementitious materials partly replace cement in the mix and are counted within the total cementitious content, not treated as a separate ingredient. The two most common SCMs used in India are fly ash, a by-product of coal power generation used per IS 3812, and GGBS (ground granulated blast-furnace slag), a by-product of iron-making.
Typical replacement levels
Replacement levels are guidance and depend on the material and the project — they should be confirmed by trial. As a typical range, fly ash is commonly used at about 15–35% of the cementitious content, and GGBS at higher levels, commonly about 25–70%.
Benefits and the early-strength trade-off
Using fly ash or GGBS typically brings lower heat of hydration, improved long-term durability, and often lower cost and embodied carbon. The main caution is slower early-age strength gain, so early-strength requirements and curing need to be checked, and the exact replacement level confirmed by trial before use.
How this fits the calculator
In the calculator, an SCM is entered as a mineral admixture — a replacement percentage of the total cementitious content — so the water-cement ratio and cementitious content calculations both account for it.
Frequently asked questions
Do SCMs replace cement completely?
No. They replace only part of the cement and are counted within the total cementitious content, not used on their own.
Do they reduce strength?
Early-age strength is usually slower to develop. Long-term strength and durability are often as good as, or better than, plain OPC concrete, but this should be confirmed by trial for your materials and mix.